This invention relates to an improved feed distributor for glassed steel wiped film evaporators.
Wiped film evaporators are used to increase evaporation or separation rates in a variety of concentrating, separating, refining, decolorizing and deoderizing processes. Typical applications include evaporation of organic compounds, concentration of solids in solution, removal of color bodies from materials of medium and high molecular weights, purification of drugs and food concentration. In these evaporators, the materials to be treated typically flows down the inside of a cylindrical vessel wall, while rotating wiper blades spread the material across the wall and maintain a uniform film of the material on the wall. Typical examples of this type of evaporator are disclosed in U.S. Pat. Nos. 2,955,990 - Smith, 3,060,107 - Smith, 3,395,419 - Nunlist et al and 3,396,085 - Nunlist
Most present wiped film evaporators use alloys such as 300 Series stainless steel for vessel walls, agitator support assemblies, feed distributors and other components that are in contact with the material being treated and/or one or more products of the process. However, in some process which involve higher temperatures and/or more corrosive products, such as some herbicides, or products with especially high purity requirements, it is desirable to have higher corrosion resistance and/or materials of construction that are easier to clean and less likely to contaminate the product. In this type of application, glass or enamel lined steel and/or various non-metallic materials such as fluorocarbons are generally more desirable materials of construction than alloys. One example of an evaporator using these materials is provided by United Kingdom Pat. Application 51576/76, filed Dec. 10, 1976 in the name of James Mitchell et al; and in corresponding U.S. Pat. application No. 817,003, filed July 18, 1977.
In many alloy wiped film evaporators, such as the one shown in Smith Pat. No. 2,955,990 and both of the above identified Nunlist patents, the incoming material to be treated flows onto a substantially flat distributor plate that rotates with the wiper assembly. From there, the material flows through short distributor pipes or nozzles (Smith) or V-shaped notches (Nunlist) onto the vessel wall, preferably just ahead of wiper blades immediately below the plate. This type of feed distributor plate serves to degas the incoming raw material and supply it evenly to the vessel wall. Also, it can be fabricated relatively easily and reasonably economically from commercially available alloy materials. Unfortunately, this type of distributor plate cannot be fabricated with present technology from glassed steel and/or fluorocarbon type plastics.
If the foregoing type of distributor were constructed of glassed steel, the high temperatures of the enamelling process would be likely to distort the critical surfaces of the plate. Subsequent machining or grinding could not adequately correct its trueness. Additionally, exposed metal would be unavoidable if any tapped holes were required. For most sizes of wiped film evaporators, distributor plates of this type cannot be formed of corrosion resistant non-metallic materials because pieces of this size are not available. A large flat plate constructed of smaller pieces of non-metallic materials is impractical because of the lack of reliable methods of joining them together.
As a result of the problems posed by glassed steel and/or non-metallic construction, previous glassed steel wiped film evaporators have used different types of feed distributors. The evaporator shown in the above-identified Mitchell et al application, for example, uses two small polytetrafluoroethylene plates fixed to the vessel wall. This type of distributor does not spread the feed evenly around the vessel. With high viscosity materials, in particular, this can result in the failure to wet part of the shell, which reduces the effective evaporation area, and may cause some of the material to be burned onto the vessel wall or otherwise degraded.